RNA viruses have been found to be valuable tools in the phenotypic and genotypic transformation of targeted cells and tissues. See, e.g., U.S. Pat. No. 5,500,360, which teaches novel viral RNA expression vectors. It has been shown that the RNA of the genome of an RNA virus can be modified to include an exogenous RNA segment and that the modified RNA can be introduced into a host cell, replicated therein, and thereby express the exogenous RNA segment.
Current methods of inoculating a host cell with modified RNA viruses involve the in vitro transcription of a particular strand followed by the introduction of the resulting RNA transcripts into the host cell. One problem with the current inoculation method is that the RNA rapidly degrades which causes a low efficiency of infection. In addition, the preparation of the in vitro RNA transcripts is expensive and time consuming.
Further, with the advent of transformation and the genetic engineering of plants, much concern has arisen concerning the potential hazard of the dispersal of dangerous traits into the environment. For example, genes increasing the stress tolerance and/or herbicide resistance of an agriculturally important crop could theoretically “leak” to surrounding less desirable and damaging plants, e.g., through pollen, mechanical or insect dispersal. This phenomenon could create a novel species of “super-weed” which could wreak havoc on the agricultural industry. Existing RNA virus-based vectors can spread to non-target plants by mechanical means and/or by insects. Such spread can be prevented by using vectors that can replicate and/or move only in target plants expressing the appropriate trans-acting factors. Accordingly, there remains a need for less expensive and more efficient methods of transformation of target cells and tissues. Moreover, there is a need for a novel method of transformation which alleviates the potential dangers associated with the unwanted spread of engineered traits into the environment.